Images as Planes leaving white areas of images looking gray
Colors are directly relative to light, your "white" value is determined by the color of the texture but by the light too (colors are just parts of light spectrum not absorbed).
For the diffuse part, RGB of 0.8 means that the surface diffuse 80% of the light it recieved.
For exemple pure red means (in RGB values) that green and blue are absorbed R=1 G=0 B=0.
Just to let you know there isn't any materials that returns 100% of the light it recieved.

3D rendering needs knowledge about light and photographie, I recommande you to read about this two topics.
There is a good doc about physically based rendering by Allegorithmic that you can read too.  

Edit your question to add pictures if you can, I don't remember if you have the right when you start here.